Optical fiber typically contains a high-purity silica glass core optionally doped with a refractive index-raising element (such as germanium), an inner cladding of high-purity silica glass optionally doped with a refractive index-lowering element (such as fluorine), and an outer cladding of undoped silica glass. In some manufacturing processes, the preforms for making such fiber are fabricated by using a glass tube for the outer cladding (referred to as an overcladding tube), and separately forming a core rod containing the core and inner cladding material. The core rods are fabricated by any of a variety of vapor deposition methods known to those skilled in the art, including vapor axial deposition (VAD), outside vapor deposition (OVD), and modified chemical vapor deposition (MCVD). MCVD, for example, involves passing a high-purity gas, e.g., a mixture of gases containing silicon and germanium, through the interior of a silica tube (also referred to as a “substrate tube”) while heating the outside of the tube with a traversing oxy-hydrogen torch. In the heated area of the tube, a gas phase reaction occurs that deposits particles on the tube wall. This deposit, which forms ahead of the torch (referred to as “downstream”), is sintered as the torch passes over it. The process is repeated in successive passes until the requisite quantity of silica and/or germanium-doped silica is deposited. Once deposition is complete, the body is heated to collapse the substrate tube and obtain a consolidated core rod in which the substrate tube constitutes the outer portion of the inner cladding material. To obtain a finished preform, the overcladding tube is typically placed over the core rod, and the components are heated and collapsed into the final preform structure.
As an alternative to MCVD, a plasma chemical vapor deposition (PCVD) process may be used. In the PCVD method, the substrate tube passes through a microwave applicator (also referred to as an activator chamber, or activator head), which forms an electro-magnetic field around and inside the tube. A non-isothermal, low-pressure plasma is generated inside the tube by the interaction of the electro-magnetic field with the feed (e.g., SiCl4, GeCl4, and O2). Chemical reactions are then enabled to form glass molecules that deposit themselves on the inside of the tube. An external heat source (such as a furnace) is required in PCVD to heat the substrate during deposition to ensure that the deposited glass is of a form that can be subsequently fused to clear glass. Once deposition is complete, the body is heated to collapse the substrate tube (in a manner similar to MCVD) and obtain a consolidated core rod in which the substrate tube constitutes the outer portion of the inner cladding material. To obtain a finished preform, an overcladding tube is typically placed over the core rod, and the components are heated and collapsed into the final preform structure.
These current methods of providing deposition of the preform materials using MCVD or PCVD exhibit deposition problems resulting from the relatively long deposition zone widths inherent in these processes (i.e., the extent of the deposition along the tube at any given instant of time). In MCVD, glass precursor vapors are introduced through a seal into the end of a substrate tube of length generally between one and three meters. The vapors encounter a reaction zone and are converted to oxides that deposit as soot on the inner tube wall. The widths of these zones of deposition are generally wider than the reaction zones and can be as much as 10-30% of the overall substrate tube length. As a result, the deposited material at the ends of the tube sometimes exhibits a non-uniform thickness, thereby adversely affecting the overall preform. Further, when multi-component compositions are being deposited (such as germanium silicates), the deposited regions tend to be nonuniform in composition as a function of zone position, due to different reaction rates of the glass constituents.
In PCVD, the reaction takes place within the created plasma region, the length of which is generally 10-20% of the overall substrate tube length. As in MCVD, there is a variation in reactivity within the plasma, resulting in variations in the thickness and/or composition of the reacted components. Therefore, the glass that is deposited on the inside of the substrate tube at any given time using a PCVD process can be non-uniform in terms of thickness and/or composition.
Moreover, the deposited cores in preforms made by these processes may vary in diameter and optical properties along the deposited length, also affecting the quality of the resulting fiber. Further, in MCVD, the soot formed in the reaction zone is capable of traveling the length of the tube and can potentially deposit itself at any point along the tube, regardless of the reaction zone location, thus leading to a certain degree of unpredictability in the deposition process.
In view of these deficiencies, there exists a need to improve the quality of materials deposited within substrate tubes.